Austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance, and method for producing austenitic stainless steel material

ABSTRACT

An austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance, comprising: C: 0.005 wt % or less; Si: 0.5 wt % or less; Mn: 0.5 wt % or less; P: 0.005 wt % or less; S: 0.005 wt % or less; Ni: 15.0 to 40.0 wt %, Cr: 20.0 to 30.0 wt %, N: 0.01 wt % or less; O: 0.01 wt % or less; and the balance of Fe and inevitable impurities, wherein the content of B included in the inevitable impurities is 3 wt ppm or less.

TECHNICAL FIELD

The present invention relates to an austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance even under severe intergranular corrosion environments such as a corrosion environment for a boiling heat transfer surface in a high-concentration nitric acid solution containing highly oxidizing metal ions and an environment in high-temperature high-pressure water under neutron irradiation, and relates to a method for producing an austenitic stainless steel material excellent in intergranular corrosion resistance and stress corrosion cracking resistance even under the severe intergranular corrosion environments.

BACKGROUND ART

It has been well known that austenitic stainless steels generally show corrosion resistance even in environments containing strong oxidizing acids such as nitric acid by forming a passive film on the surface. The austenitic stainless steels have been used, for example: as a structural material in nitric acid production plants; and as a material for dissolvers for dissolving the spent nuclear fuels with high-concentration nitric acid and acid recovery evaporators for recovery of nitric acid by distillation of the solution in the dissolver in plants for reprocessing spent nuclear fuels. The austenitic stainless steels are also used as a material for light water reactor cores under the environment in high-temperature high-pressure water under neutron irradiation.

When an austenitic stainless steel material is used as a material for the dissolver or the acid recovery evaporator in plants for reprocessing spent nuclear fuels, metal ions such as cerium ion (Ce⁴⁺), ruthenium ion (Ru³⁺ and chromium ion (Cr⁶⁺) are released into the nitric acid from the spent nuclear fuels and thus, the nitric acid solution becomes more oxidative than that in the nitric acid production plant. For that reason, the austenitic stainless steel is more susceptible to corrosion accompanied by intergranular corrosion.

The following measures are taken, for use of an austenitic stainless steel material under environments of high-temperature nitric acid containing highly oxidizing metal ions: First, the carbon content in the austenitic stainless steel is reduced as much as possible, for prevention of generation of Cr-depletion layer, which is the cause of intergranular corrosion. Nb is added as needed in small amount to the austenitic stainless steel. In addition, the austenitic stainless steel material is subjected to solution treatment.

Methods of improving the corrosion resistance of austenitic stainless steels include, for example, those described in Patent Documents 1 to 7.

Patent Document 1 disclosed an austenitic stainless steel containing C at 0.005 wt % or less, Si at 0.4 wt % or less, Mn at 0.1 to 12 wt %, P at 0.005 wt % or less, Ni at 7 to 28 wt %, Cr at 15 to 30 wt %, N at 0.06 to 0.30 wt % and the balance of essentially Fe. The intergranular corrosion resistance of the austenitic stainless steel is improved, by suppressing aggregation of P in grain boundaries, while the P content in the austenitic stainless steel is restricted.

Patent Document 2 discloses an austenitic stainless steel containing C at 0.015 wt % or less, Si at 0.5 wt % or less, Mn at 2 wt % or less, P at 0.015 wt % or less, Ni at 10 to 22 wt %, Cr at 15 to 30 wt %, Al at 0.01 wt % or less, Ca at 0.002 to 0.010 wt % or less and the balance of essentially Fe. Favorable process-flow corrosion resistance is obtained while the contents of Si, P and Al in the austenitic stainless steel are restricted and Ca is added in suitable amount. The austenitic stainless steel also shows favorable hot-processing efficiency and favorable corrosion resistance in high-temperature nitric acid.

Patent Document 3 discloses an austenitic stainless steel containing C at 0.02 wt % or less, Si at 0.5 wt % or less, Mn at 0.5 wt % or less, P at 0.03 wt % or less, S at 0.002 wt % or less, Ni at 10 to 16 wt %, Cr at 16 to 20 wt %, Mo at 2.0 to 3.0 wt %, N at 0.06 to 0.15 wt % and the balance of essentially Fe. The austenitic stainless steel, which satisfies the formula: [Ni]+60[N]−4[Mo]≧7 and contains one or both of Ca and Ce in an individual or total amount of 2×[S] to 0.03 (wt %), shows favorable corrosion resistance to tunnel-like corrosion. The symbol [X] represents the content of element X in steel (wt %).

Patent Document 4 discloses a method of producing an austenitic stainless steel resistant to corrosion by high-temperature nitric acid containing oxidizing metal ions. Specifically, the stainless steel is heat-treated at a temperature in the range of 650 to 950° C. for 1 minute or more. Then, if the heat-treatment temperature is lower than the range of 650 to 850° C., the stainless steel is cooled to ordinary temperature by rapid or natural cooling. Alternatively if the heat-treatment temperature is higher than the range of 850 to 950° C., the stainless steel is cooled to ordinary temperature by rapid cooling. The austenitic stainless steel prepared in this way shows favorable resistance to corrosion by high-temperature nitric acid.

Alternatively, Patent Document 5 disclosed a method of producing an austenitic stainless steel containing B at 30 wt ppm or less and satisfying, when the diameter of the austenitic grains is designated by d, the following formula: B (wt ppm)×d (μm)≦700. The austenitic stainless steel, when subjected to solution treatment, as heated at a particular temperature, which is a function of B (wt ppm)×d (μm), or higher, shows excellent intergranular corrosion resistance and intergranular stress corrosion cracking resistance.

Patent Document 6 discloses an austenitic stainless steel containing C at 0.02 wt % or less, Si at 0.8 wt % or less, Mn at 2.0 wt % or less, P at 0.04 wt % or less, S at 0.03 wt % or less, Ni at 6 to 22 wt %, Cr at 13 to 27 wt %, Al at 0.1 wt % or less, Cu at 0.3 wt % or less, N at 0.1 wt % or less and the balance of essentially Fe. The austenitic stainless steel, which satisfies the following formula: 1.5 [Ni]+[Mn]+65([C]+[N])·5[Si]−2.5≦52−2.3 ([Ni]+[Mn])−200([C]+[N]) and contains B in an amount of 5 wt ppm or less, shows favorable resistance to nitric acid corrosion after cold processing or deformation, when it has a total content of one or more elements selected from Ti, Nb, V, Hf and Ta at 1.0 wt % or less.

Patent Document 7 discloses a method of producing an austenitic stainless steel by forming clean grain boundaries. Specifically, the austenitic stainless steel is cold worked to a working ratio of 40% or more. Then, the stainless steel obtained after cold working is kept at a temperature that is lower than the recrystallization temperature and allows carbide precipitation, for recrystallization in a temperature range prohibiting segregation of P and other elements in grain boundaries. After the treatment, the austenitic stainless steel shows favorable corrosion resistance even corrosive environment of nitric acid solution containing oxidants.

On the other hand, when the austenitic stainless steel material is used in a light water reactor core under an environment in high-temperature high-pressure water exposed to neutron irradiation, it becomes more susceptible to intergranular stress corrosion cracking (IGSCC) by long-term irradiation. For example, a solution-treated austenitic stainless steel in solid-solution state shows resistance to intergranular stress corrosion cracking outside the reactor core where no neutron is irradiated, but such resistance is lost when it is exposed to high level of irradiation, in particular to fluence equivalent to or more than about 1.0×10²¹ n/cm² or more in the reactor core. Such cracking, which is also called irradiation-assisted stress corrosion cracking (IASCC), is a concern recently in old light water reactors.

As means to solve the problem above, for example, Patent Documents 8 and 9 disclose methods of adjusting the elements constituting the austenitic stainless steel. Alternatively, Patent Document 10 discloses a steel that is lower in Cr deficiency in grain boundaries and has dispersed Cr-depletion regions, which is prepared by reducing the amount of carbide precipitation per unit grain boundary by means of heating a Ni-Cr austenitic stainless steel that have a C content restricted to 0.03 wt % or less and a content of N, which is higher in solid solution-forming efficiency, adjusted to 0.15 wt % or less in its chemical composition for prevention of precipitation of carbides, a cause of intergranular stress corrosion cracking, in grain boundaries in a temperature range of 1100 to 1300° C., and a preparative method thereof.

Patent Document 11 discloses a high-Ni-content austenitic steel excellent in neutron deterioration resistance prepared by subjecting an austenitic stainless steel in the chemical composition containing C at 0.005 to 0.08 wt % or less, Mn at 0.3 wt % or less, Si+P+S at 0.2 wt % or less, Ni at 25 to 40 wt %, Cr at 25 to 40 wt %, Mo+W at 5.0 wt % or less, Nb+Ta at 0.3 wt % or less, Ti at 0.3 wt % or less, B at 0.001 wt % or less and others, to solution treatment in a temperature range of 1000 to 1150° C., cold working to its ratio of 30%, and heat treatment in a temperature range of 600 to 750° C. for 100 hours. The high-Ni-content austenitic steel shows favorable stress corrosion cracking resistance in high-temperature high-pressure water or high-temperature high-pressure oxygen-saturated water at 270 to 350° C. and at 70 to 160 atmospheric pressures even after neutron irradiation at a fluence of at least 1×10²² n/cm². The average expansion coefficient of the high-Ni-content austenitic steel in the temperature range of room temperature to 400° C. is in the range of 15 to 19x10⁻⁶ /K.

Patent Document 12 discloses a high-alloy austenitic stainless steel containing C at 0.05 wt % or less, Si at 1.0 to 4.0 wt %, Mn at 0.3 wt % or less, Ni at 6 to 22 wt %, Cr at 18 to 23 wt %, Cu at 1 to 3 wt %, Mo at 0.3 to 2.0 wt %, N at 0.05 wt % or less, S at 0.004 wt % or less, a small amount of B added at 0.0005 to 0.005 wt %, one or both of Ca and Mg added in an amount of [S]≦[Mg]+½ and [Ca]≦0.007, and the balance of essentially Fe. It is disclosed that the austenitic stainless steel is improved significantly in processing efficiency without deterioration in its favorable corrosion resistance.

Patent Document 13 discloses a method of preparing single crystal by removing random grain boundaries in austenitic stainless steel by unidirectional solidification method.

Patent Document 14 discloses an austenitic stainless steel containing C at 0.02 wt % or less, N at 0.6 wt % or less, Si at 1.0 wt % or less, P at 0.040 wt % or less, S at 0.030 wt % or less, Mn at 2.0 wt % or less, Mo at 3.0 wt % or less, Ni at 12 to 26 wt % and Cr at 16 to 26 wt %, in which austenite or ferrite phases are contained in the austenitic matrix in an amount of 10 vol.% or less at room temperature, the matrix contains sub-crystalline grains, and single-crystalline grain boundaries smaller in deviation from the coincidence orientation relationship and higher in crystal orientation are formed. The austenitic stainless steel shows favorable corrosion resistance and stress corrosion cracking resistance and has superior mechanical properties.

-   Patent Document 1: JP-A No. 59-222563 -   Patent Document 2: JP-A No. 06-306548 -   Patent Document 3: JP-A No. 07-090497 -   Patent Document 4: JP-A No. 07-238315 -   Patent Document 5: JP-A No. 07-113146 -   Patent Document 6: JP-A No. 08-013095 -   Patent Document 7: JP-A No. 60-100629 -   Patent Document 8: JP-A No. 63-303038 -   Patent Document 9: JP-A No. 05-059494 -   Patent Document 10: JP-A No. 8-269550 -   Patent Document 11: JP-A No. 09-125205 -   Patent Document 12: JP-A No. 05-179405 -   Patent Document 13: JP-A No. 03-264651 -   Patent Document 14: JP-A No. 11-80905

SUMMARY OF THE INVENTION

When an austenitic stainless steel is used in an acid recovery evaporator by thermosiphon process, in which nitric acid is recovered by distillation of a nitric acid solution in heat-transfer pipes by application of heat from outside, generation of highly oxidizing ions associated with distillation of nitric acid and thermal decomposition and solubilization by reductive reaction occur at the same time. Thus, the corrosion environment to the austenitic stainless steel is in a boiling-inducing heat transfer surface corrosion. It is a severe environment in which the corrosion rate is higher than that by immersion corrosion at the same metal surface temperature and the corrosion rate increases gradually over time. For that reason, even if the austenitic stainless steel materials described in Patent Documents 1 to 7 or the preparation methods thereof are used, there still remains a possibility of severe intergranular corrosion.

Specifically, Patent Document 1 discloses that it is possible to suppress formation of MnS and thus generation of the tunnel-like corrosion caused by MnS that is extending in the rolling direction by restricting the P content, and Patent Documents 2 and 3, by adding Ca and Ce that have strong binding force to S. However, these documents only describe that suppression of segregation of S in grain boundaries is effective in preventing intergranular corrosion, but do not contain any specific description. In addition, Patent Documents 4 and 5 only consider economy, and the stainless steels cannot be considered to be resistance to nitric acid corrosion consistently.

Patent Document 6 discloses an austenitic stainless steel containing B at 5 wt ppm or less and one or more elements selected from Ti, Nb, V, Hf and Ta in a total amount of 1.0 wt % or less. However, the test in Patent Document 6 was carried out under a mild corrosive condition of immersion only in boiling 65% nitric acid for 48 hours. The test is an evaluation test simulating the corrosion environment containing highly oxidizing metal ions that is used in reprocessing plants for spent nuclear fuels, and is not suited for evaluation of advantages and disadvantages in corrosion resistance of stainless steels. As for B content, it is only described that the B content is desirably lower, similarly to other normal impurity elements, and because the B contents in the austenitic stainless steels were at the same level between Examples and Comparative Examples, and there is no description that the B content should be adjusted.

Patent Document 7 discloses a thermomechanical treatment of cold working a steel material to a working ratio of 40% or more, recrystallizing it by keeping it at a temperature in the temperature range lower than the recrystallization temperature but allowing precipitation of carbides and prohibiting segregation for example of P in grain boundaries, but the C content in the steel was not specified sufficient. Thus, after the cold working, Cr based carbides, a possible cause of intergranular corrosion, are dispersed uniformly, but the Cr-depletion layers formed around the Cr based carbides precipitating in great amounts lead to acceleration of corrosion. In addition, the heat treatment is not effective at all for removal of the impurity elements, such as P, S, N and O, segregating in grain boundaries. In the method described in Patent Document 7, the amounts of the impurity elements, such as P, segregating in grain boundaries are not described sufficiently, and no measure is taken for their removal. Accordingly, the method unlikely gives desired corrosion resistance.

Unfavorably when the austenitic stainless steel material is used in a light water reactor core under an environment in high-temperature high-pressure water exposed to neutron irradiation, the austenitic stainless steel materials described in Patent Documents 8 to 13 did not have sufficient corrosion resistance.

Specifically, the methods described in Patent Documents 8 to 10 do not reduce the content of impurities, a cause of intergranular corrosion together with Cr-depletion layer, for prevention of the intergranular stress corrosion cracking by component adjustment. It is thus impossible in principle to solve the problem of stress corrosion cracking generated under the irradiation environment.

The method described in Patent Document 11 specifies that the contents of elements P, S, Si, Nb, Ta, Ti and B are preferably lower, and the contents of Nb, Ta and Ti are specifications when used as a deoxidizing agent, and thus, the contents are not adjusted intentionally for improvement in stress corrosion cracking resistance. As for Mn and B contents, the B content is specified as 0.001 wt % or less, the lowest value practically possible by the steel making technologies at the time of invention, but the lowest value of the B content found in Examples is 0.0003 wt %. The influence exerted by a B content of less than 0.0003 wt % on stress corrosion cracking resistance is unknown. Because the C content, which is the most important constituent component leading to deterioration in stress corrosion cracking resistance, is not reduced sufficient, it is not always possible to obtain favorable stress corrosion cracking resistance.

In Patent Document 12, the lower limit value of the B content is restricted to 0.0005 wt % for improvement in hot processing efficiency and the higher limit value to 0.005 wt % for prevention of deterioration in intergranular corrosion resistance, but it is obvious that the limitation above is not sufficiently effective in improving the corrosion resistance.

The method of preparing a single crystal described in Patent Document 13 imposes restriction on casting condition, in particular on withdrawing velocity, which makes industrial application thereof, in particular application to large-sized materials, difficult.

Patent Document 14 discloses that stainless steel production methods include deformation annealing method, Tammann method, Bridgemann method, floating zone melting method, unidirectional solidification method, and continuous casting method and use of a unidirectional solidification method or a continuous casting method is preferable for production of relatively large-sized steel. However, the patent document does not specify typical manufacturing conditions, and it is doubtful that a single-crystalline metal structure having subcrystal grains can be obtained. In addition, the content of the steel components, in particular Ni, is not sufficient for prevention of swelling under the neutron irradiation environment, and it is unlikely that desired irradiation resistance is obtained.

The present invention was made to solve the problems above. It is an object of the invention to provide an austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance even under two environments: one environment is a corrosion environment for a boiling heat transfer surface in a high-concentration nitric acid solution containing highly oxidizing metal ions; and another environment is an environment in high-temperature high-pressure water exposed to neutron irradiation. It is another object of the invention to provide a method for producing an austenitic stainless steel material excellent in intergranular corrosion resistance and stress corrosion cracking resistance even under the abovementioned two environments.

According to an aspect of the present invention which achieved the object, an austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance comprises: C: 0.005 wt % or less; Si: 0.5 wt % or less; Mn: 0.5 wt % or less; P: 0.005 wt % or less; S: 0.005 wt % or less; Ni: 15.0 to 40.0 wt %, Cr: 20.0 to 30.0 wt %, N: 0.01 wt % or less; 0: 0.01 wt % or less; and the balance of Fe and inevitable impurities, wherein the content of B included in the inevitable impurities is 3 wt ppm or less.

According to another aspect of the present invention which achieved the object, an method of producing an austenitic stainless steel material excellent in intergranular corrosion resistance and stress corrosion cracking resistance comprises; a step of hot working an ingot having a chemical composition of the stainless steel; and a step of solution treating a stainless steel material obtained by the hot working; wherein the solution treatment step comprises: a substep of heating the stainless steel material at a heat treatment temperature in a first temperature range of 1000 to 1150° C. for 1 minute or more; and then, a substep of cooling the stainless steel material from the heat treatment temperature in the first temperature range to ordinary temperature by rapid or natural cooling.

According to yet another aspect of the invention which achieved the object, an method of producing an austenitic stainless steel material excellent in intergranular corrosion resistance and stress corrosion cracking resistance comprises: a step of hot working an ingot having a chemical composition of the stainless steel; and a step of solution treating a stainless steel material obtained by the hot working; wherein the solution treatment step comprises: a substep of heating the stainless steel material at a heat treatment temperature in a first temperature range of 1000 to 1150° C. for 1 minute or more; a substep of cooling the stainless steel material from the heat treatment temperature in the first temperature range by rapid or natural cooling; a substep of heating the stainless steel material at a heat treatment temperature in a second temperature range of 650° C. or higher for 10 minutes or more after the cooling, and then, a substep of cooling the stainless steel material from the heat treatment temperature in the second temperature range to ordinary temperature by rapid or natural cooling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graph showing the relationship between the corrosion rate and the B content in Coriou corrosion test.

FIG. 1B is a graph showing the relationship between the intergranular corrosion depth and the B content in Coriou corrosion test.

FIG. 2 is a view of a jig used in CBB test.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

After intensive studies to solve the problems above, the inventors have found that it is possible to improve the corrosion resistance to intergranular corrosion and stress corrosion cracking, even under severe intergranular corrosion environments such as a corrosion environment for a boiling heat transfer surface in a high-concentration nitric acid solution containing highly oxidizing metal ions, and an environment in high-temperature high-pressure water exposed to neutron irradiation, for example, in a light-water reactor core, by reducing the amount of impurities, in particular of B, present in grain boundaries of the austenitic stainless steel, which becomes the initial point of corrosion, as much as possible, and completed the present invention.

Favorable embodiments of the present invention, including the reasons for the alloy design specified, will be described below with reference to FIGS. 1A, 1B and 2.

First, it was found that it was not possible to avoid problems such as sensitization caused by heating, for example by welding, and irradiation-induced precipitation under irradiation environment, simply by a measure of reducing the C content, one of traditional measures, for prevention of generation of Cr-depletion layer caused by precipitation of Cr based carbides in grain boundaries, which is the most important factor for generation of intergranular corrosion and stress corrosion cracking.

Specifically, if the Cr content in steel is 20 wt % or more, the Cr content remains at about 12 wt %, which is needed for generation of a passive film, even after generation of the depletion layer by carbide precipitation. However, it was not possible to avoid the overpassivation corrosion completely under a corrosion environment for a boiling heat transfer surface in a high-concentration nitric acid solution containing highly oxidizing metal ions and to avoid grain boundary damages under an environment in high-temperature high-pressure water exposed to neutron irradiation.

One of the reasons for it would be segregation of an impurity element B in grain boundaries, leading to decrease in grain-boundary bonding energy. Thus, as shown in FIGS. 1A and 1B, the relationships between the corrosion rate and the intergranular corrosion depth with the B content were studied, and it was found that it is possible to control the intergranular corrosion and the stress corrosion cracking by reducing the B content to 3 wt ppm or less.

It is known that addition of B leads to improvement in high-temperature ductility of austenitic stainless steels. For example, JP-A No. 63-069947 proposed a method of improving the creep rupture ductility by adding 6 to 25 wt ppm of B. Alternatively, “Iron Age” vol. 179 (1957), p.95 reported that addition of B in an amount of 2 wt ppm or more was effective in improving the hot ductility. Thus, B is said to be an element effective in improving the high-temperature ductility and hot processing efficiency. However on the other hand, it was reported that addition of B lead to deterioration in corrosion resistance of austenitic stainless steels.

“Stainless steel '87”, The Institute of Metals, London, (1987), p.234 proposed reduction in B content for preservation of the intergranular corrosion resistance of austenitic stainless steels, and reports that addition of B in an amount of approximately 25 wt ppm leads to precipitation of Cr borides in grain boundaries and thus to deterioration in intergranular corrosion resistance even in normal solution treatment. In addition, “Materials and Processes”, Iron and Steel, vol.6 (1993), p.732 reports that the B content should be reduced to 9 wt ppm or less, for preservation of the intergranular corrosion resistance of austenitic stainless steels at high level in high-temperature high-concentration nitric acid solution. As described above, B is known to segregate in grain boundaries, form Cr-rich boride, and thus, reduce the intergranular corrosion resistance. Thus, prior art including the method disclosed in Patent Document 7 teaches that, in steels at conventional impurity level, a B content of more than 5 wt ppm shows at least an adverse effect leading to deterioration in intergranular corrosion resistance, and a B content of more than 10 wt ppm, to particularly drastic deterioration.

Problems caused by addition of B are as described above, but the inventors have found that it is important to reduce the B content further. Although the reason for it is not perfectly clear, a content of B below the solubility limit of B in grain boundaries, which is estimated to be about 10 wt ppm, leads to distinctive improvement in preventing grain boundary damage. This may lead that solid solution formation in grain boundaries themself, rather than boride formation, has adverse effects. The discovery of the effect of such a trace content of B in the present invention is largely dependent on recent progress in analytical instruments and methods and steel-making technology. The detection limit in conventional chemical analysis was about 2 wt ppm, but it is now possible to analyze B content in the order of wt ppm or less and to elucidate the relationship between a trace content of B and intergranular corrosion or stress corrosion cracking, by GD-MS analysis. Contamination at about 2 to 5 wt ppm from raw materials such as alloy irons and scraps was inevitable in ingots of normal austenitic stainless steels, but it became possible to select a raw material lower in B content by progress of analysis technology and additionally to produce an ingot of low austenitic stainless steel lower in B content by progress of the steel-making technology such as oxidizing refining.

One of the causes for the overpassivation corrosion under the corrosion environment for the boiling heat transfer surface and the grain boundary damage under the environment in high-temperature high-pressure water would be impurity elements such as C, P, S, N, O segregating in grain boundaries. It was also found that, when the B content is 3 wt ppm or less, it was possible to prevent the intergranular corrosion and the stress corrosion cracking by adjusting the total content of the impurity elements such as C, P, S, N and O to 0.02 wt % or less. The reason for the drastic increase in grain boundary damage when the total content of the impurity elements is 0.02 wt % or less is yet to be understood. Although the actions to grain boundaries by these elements and the states of the deposits when formed may vary, it is impossible to determine the states of trace amounts of the elements in the present invention separately by currently available analytical technology. However, there would be no doubt about the impurity elements segregating and present as solid solution in grain boundaries exerting adverse effects.

It was found that, for elimination of the influence by the impurity elements such as C, P, S, N and O as needed, it is effective to add Ti in an amount stoichiometric to or more than the total amount of C, P, S, N and O and thus to precipitate these impurities as Ti based carbides, nitrides and other compounds such as TiC, TiN, FeTiP, TiS and TiO₂. It was possible in this way to suppress the intergranular corrosion and the stress corrosion cracking further.

The impurity elements segregating in grain boundaries such as C, P, S, N and O are desirable lower in amount, but it is difficult and also uneconomical to eliminate these impurities completely by current refining technology. It is effective to add a stabilizing element (element immobilizing impurity elements) for elimination of such impurity elements as much as possible, but Ti is most desirable for elimination of these impurity elements. Addition of Ti allows conversion of impurities such as C, P, S, N and O that could not be removed, for example by the electrobeam-melting method described below, to Ti based compounds such as TiC, FeTiP, TiS, TiN and TiO₂ and control of segregation of the impurities in grain boundaries as solid-solution elements. For example, Nb is used as the stabilizing element in traditional technology, but it is difficult to form the compounds other than NbC by addition of Nb when the austenitic stainless steel contains impurities in the range of the present invention, and thus, the advantageous effects obtained by addition of Nb is limited. The amount of Ti added should be stoichiometrically equivalent to or more than the total amount of C, P, S, N and O.

It was also found that it was possible to prevent the intergranular corrosion and the stress corrosion cracking by uniformizing the austenite grains and dispersing carbides and other deposits uniformly by means of heat-treating the austenitic stainless steel material in the chemical composition above in the production process. For further improvement of intergranular corrosion resistance and stress corrosion cracking resistance, it is effective to heat the plate or pipe material in a temperature range of 1000 to 1150° C. for 1 minute or more in its production process (for example, in the hot working step); and then, to cool it from the heat treatment temperature to ordinary temperature, or to keep the heated material consistently at a temperature in the range of 650° C. or higher for 10 minutes or more during cooling or by reheating. In addition, the material is heated in the temperature range of 1000 to 1150° C. for 1 minute or more; and then, cooled rapidly or cooled by natural air from the heat treatment temperature to ordinary temperature, to make the Ti-addition effect more definite and also to make the relationship between the distribution state of the Ti based compounds generated and the position of grain boundaries present different. It is also effective to cold work the material, after solution treatment, to a working ratio of 40% or more and less than 75%; and then, to heat and hold the cold worked material at a temperature in the temperature range of 750° C. or higher for 10 minutes or more for recrystallization. Because the precipitation reaction above may possibly proceed insufficiently from the point of reaction rate in a material having a chemical composition lower in the amount of reactive impurity elements such as C, P, S, N and O, as in the present invention, it is also effective to cold work the material to a working ratio of 40% or more and less than 75%, apply deposit precipitation treatment under strain aging on it by heating and holding it in a temperature range of 500 to 650° C. for 30 minutes or more, and then, heat and hold it in a temperature range of 750° C. or higher for 10 minutes or more.

(Chemical composition of stainless steel) C: 0.005 wt % or less

Carbon gives Cr based carbide precipitated in grain boundaries, when an austenitic stainless steel is heat-treated or welded. It gives Cr-depletion regions formed close to grain boundaries. When the stainless steel material is placed in a corrosive environment in the state, intergranular corrosion, i.e., selectively corrosion in the region, occurs. Thus, it leads to deterioration in nitric acid corrosion resistance and stress corrosion cracking resistance of the austenitic stainless steel. Although the impurities are removed by addition of Ti and thermomechanical treatment in the present embodiment, because, if the C content in the austenitic stainless steel is larger, it may possibly leads to microscopic precipitation of Cr based carbides, the content of C is preferably 0.005 wt % or less, more preferably 0.003 wt % or less.

Si: 0.5 wt % or less

Silicon is effective as a deoxidizing agent, the Si content is 0.5 wt % or less. However, for prevention of intergranular corrosion, the content is desirably smaller, and it is more preferably 0.3 wt % or less.

Mn: 0.5 wt % or less

Manganese is effective in improving the stability of the austenite phase and in preventing generation of δ-ferrite and processing-induced phase transformation which are hazardous to corrosion resistance, but does not exhibit desired effects even at a content of more than 0.5 wt % and rather accelerates corrosion, as Mn in the solid-solution state, and thus, the Mn content is 0.5 wt % or less. It is more preferably 0.3 wt % or less.

P: 0.005 wt % or less

Phosphorus is known to segregate in grain boundaries, and increase in the P content leads to deterioration in intergranular corrosion resistance and stress corrosion cracking resistance. Thus, the P content is desirably lower and preferably 0.005 wt % or less, more preferably 0.003 wt % or less.

S: 0.005 wt % or less

Increase in the content of sulfur leads to acceleration of sulfide production and, by selective corrosion based on the sulfide, to deterioration in intergranular corrosion resistance, stress corrosion cracking resistance as well as production lot number. Thus, the S content is desirably lower, and preferably 0.005 wt % or less, more preferably 0.003 wt % or less.

Ni: 15.0 to 40.0 wt %

Nickel is an element needed for stabilization of austenite structure and also for control of intergranular corrosion and stress corrosion cracking. However, the Ni content of less than 15 wt % cannot assure generation of the austenite structure sufficiently and acquisition of swelling resistance under neutron irradiation environment. On the other hand, the Ni content of more than 40 wt % leads to increase in price and the Ni content is thus desirably 15.0 to 40.0 wt %. It is more preferably 18.0 wt % or more, from the point of stability of the austenite structure. It is more preferably 38.0 wt % or less for control of swelling.

Cr: 20.0 to 30.0 wt %

Chromium is an element needed for formation of a passive film and thus for assuring corrosion resistance of the stainless steel. For formation of passive film, the Cr content is preferably about 16%, as in typical stainless steels specified in JIS standard such as SUS304 and SUS316. However, the Cr content should be at least 20 wt % for assuring sufficient corrosion resistance under an overpassivation corrosion environment under a corrosion environment for a boiling heat transfer surface in a high-concentration nitric acid solution containing highly oxidizing metal ions such as in reprocessing plants, and under an environment in high-temperature high-pressure water exposed to neutron irradiation such as in light water reactor cores. On the other hand, the Cr content of more than 30 wt % leads to precipitation of Cr-rich brittle phases, and, for generation of complete austenite structure while avoiding the precipitation, the Ni content should be raised, inevitably leading to increase in cost, and thus, the Cr content is desirably 20.0 to 30.0 wt %. It is more preferably 22.0 wt % or more from the viewpoint of corrosion resistance. It is further more preferably 28.0 wt % or less.

B: 3 wt ppm or less

Boron is the factor most important in the configuration of the present invention. It is fundamentally an impurity element that segregates in grain boundaries and causes deterioration in intergranular corrosion resistance and stress corrosion cracking resistance, and thus, the content thereof is preferably lower as much as possible. It was not possible to determine a B content of 0.0003 wt % or less by conventional analytical methods. However, the inventors have identified the relationship between the trace concentration of B and the corrosion resistance by using recent analytical methods and, as a result, found that it was possible to prevent the intergranular corrosion and the stress corrosion cracking completely by reducing the concentration thereof to 0.0003 wt % or less. Thus, the B content is 3 wt ppm (0.0003 wt %) or less from the viewpoint above. It is more preferably 1.5 wt ppm or less.

N: 0.01 wt % or less O: 0.01 wt % or less

Nitrogen and Oxygen are both elements that leads to deterioration in intergranular corrosion resistance and stress corrosion cracking resistance, and thus, the content thereof is preferably lower, and the content of each element is 0.01 wt % at most. The N content is more preferably 0.005 wt % or less. The O content is more preferably 0.005 wt % or less.

C+P+S+O+N: 0.02 wt % or less

Even when the contents of impurity elements of C, P, S, O and N are adjusted respectively under the regulation conditions above, it is not possible to have favorable intergranular corrosion resistance and stress corrosion cracking resistance if the total content of these elements exceeds 0.02 wt %, and thus, the upper limit of the total content is 0.02 wt %. The upper limit is more preferably 0.015 wt %.

The method of forming the ingot of a high-purity austenitic stainless steel having a total impurity element content of 0.02 wt % or less is not particularly limited, but one of effective means is to use an electrobeam-melting method in the combination of ingot-forming step. It is possible by employing the electrobeam-melting method in the production process of austenitic stainless steel ingot, to obtain ultrahigh-cleanness steel ingot reduced in the contents of the impurity elements segregating in austenite grain boundaries, such as C, P, S, N and O, and also of highly volatile alkali group metals. The method of forming the raw electrode for electrobeam melting is not particularly limited, and a most suitable electrode forming method is selected according to the purity of the raw materials for primary melting.

Ti: An amount stoichiometrically equivalent to or more than the total content of C, P, S, N and O.

Titanium is a factor important in the configuration of the present invention and is added for complete removal of the impurity elements such as C, P, S, N and 0, causes of intergranular corrosion, by conversion to Ti based carbides, nitrides and other compounds such as TiC, TiN, FeTiP, TiS and TiO₂. By adoption of the electrobeam-melting method, the contents of these impurity elements are already at en extremely lowered level in the steel ingot-forming phase. However, studies by the inventors showed that trace amounts of impurity elements not removed by electrobeam melting could exert adverse effects on intergranular corrosion. Therefore, addition of Ti is more effective for complete removal of these impurities. Accordingly, the Ti content is preferably an amount stoichiometrically equivalent to or more than the amount needed for conversion of all impurity elements of C, P, S, N and O to Ti based carbides, nitrides and other compounds such as TiC, TiN, FeTiP, TiS and TiO_(2.) Specifically, Ti (wt %)≧(48/12)C (wt %)+(48/31)P (wt %)+(48/32)S (wt %)+(48/14)N (wt %)+(48/16)×(1/2)O (wt %). Moreover, it is preferably 0.05 wt % or more, considering the dynamic precipitation reaction of dilute elements. On the other hand, addition of Ti in great amount leads to cost increase and thus, the Ti content is preferably 0.3 wt % or less.

(Electrobeam-Melting Method)

An electrobeam-melting method is employed in the steel ingot production process in the present embodiment. The electrobeam-melting methods are basically grouped into drip melting method and cold-hearth melting method. The drip melting method is a method of irradiating the edge of a raw material electrode with electron beam and spraying the generated droplets directly onto a water-cooled cylindrical mold in layer. Alternatively, the cold-hearth melting method is a method of collecting the droplets generated on the edge of a raw material once in a water-cooled shallow copper container called cold hearth and depositing the molten metal on a base plate called starting block in layer, by pouring the molten metal overflowing therefrom onto a water-cooled cylindrical mold. Any one of the melting methods may be used in the present embodiment.

The conditions required for the electrobeam-melting method will be described below. For improvement in purification effect by vaporization during melting, the vacuum in the chamber should be kept at 1×10⁻² Pa or more. However, excessive elevation of the degree of vacuum leads to vaporization of highly volatile elements including Cr, an element constituting the present invention, making it difficult to adjust the composition and lowering the industrial viability, and thus, the degree of vacuum is desirably 1×10⁻⁴ Pa or less. The raw material for the raw material electrode may be prepared, for example, by the AOD method (argon oxygen decarburization method) or the VOD method (vacuum oxygen decarburization method), both of which are widely known as the methods of forming a stainless steel ingot, or a special melting method such as vacuum induction melting method or cold crucible induction melting method.

(Production Method)

In the present embodiment, a plate or pipe material of austenitic stainless steel is heat-treated in the production process (for example, in the hot working step) at a heat-treatment temperature in the range of 1000 to 1150° C. for 1 minute or more. It is then cooled from the heat-treatment temperature in the range of 1000 to 1150° C. to ordinary temperature by rapid or natural cooling in solution treatment. In the solution treatment, the plate or pipe material may be cooled to or reheated after cooling to a heat-treatment temperature in the range of 650° C. or higher; and then, heated additionally for 10 minutes or more at a heat-treatment temperature in the range of 650° C. or higher. It is possible in this way to make the austenite phase more homogeneous and to raise the efficiency in improving the intergranular corrosion resistance and the stress corrosion cracking resistance by restriction of the chemical composition of the austenitic stainless steel.

In addition, the material may be subjected to cold working (cold rolling) and recrystallization treatment, after the solution treatment. The cold working, if carried out, can introduce dislocations, i.e., sites for carbide precipitation, in great amount. Alternatively, the recrystallization treatment by heat treatment after the cold working allows uniformly disperses precipitation and co-recrystallization of the deposits.

Hereinafter, typical cold working will be described. For sufficient introduction of dislocation sites as precipitation sites, the working ratio in the cold working is 40% or more. Excessive increase in working ratio only leads to saturation of the density of the dislocation sites introduced and distortion-induced phase transformation from the austenite phase to the martensite phase. The phase transformation makes industrial processing more difficult and prohibits generation of homogeneous austenite structure in the following recrystallization treatment, consequently leading to deterioration in intergranular corrosion resistance and stress corrosion cracking resistance. Thus, the working ratio of the cold working is less than 75%.

Hereinafter, recrystallization treatment will be described. The temperature for recrystallization of the processing structure depends on the working ratio of the steel, specifically on the density of introduced dislocation sites and the dispersion state of the carbides inhibiting movement of dislocations in the recovery-recrystallization process. For that reason, the temperature should be kept at 700° C. or higher for 10 minutes or more for the steel having the chemical compositions and structures according to the present invention. On the other hand, excessively high temperature leads to deterioration in strength, by growth of the recrystallized austenite grains obtained. Further, the deposits aggregate and form coarse particles, which are distributed in the recrystallized austenite grain boundaries. It thus leads to deterioration in intergranular corrosion resistance and stress corrosion cracking resistance. Accordingly, the recrystallization temperature is desirably 900° C. or lower.

The precipitation treatment may be carried out for efficient and uniform dispersion of the carbides after cold working and before recrystallization treatment. Theoretically, it is desired then to heat the material at a constant temperature in the range of 500° C. or higher for 30 minutes or more. On the other hand, high temperature shortens the period needed for carbide precipitation, but excessively high temperature leads to recovery and recrystallization before carbide precipitation. It prohibits precipitation at the dislocation sites once introduced and uniform dispersion of the carbides, because of preferential precipitation in grain boundaries, and thus leads to further growth of the grains. It consequently prohibits superior intergranular corrosion resistance and stress corrosion cracking resistance. From the viewpoints above, the carbide precipitation treatment is preferably carried out, as the stainless steel is heated consistently at a temperature in the range of 500 to 650° C. for 30 minutes or more.

EXAMPLES

Hereinafter, the present invention will be described more specifically with reference to examples. It should be understood that the present invention is not restricted by the Examples at all.

(Experiment 1)

150 kg of an austenitic stainless steel in the chemical compositions shown in Table 1 was melted by vacuum induction melting (VIM) and cast into a mold, to give an ingot. The units for elements in the chemical compositions shown in Table 1 are respectively weight percentage (wt %), except the unit for B of wt ppm. Then, an electrode was prepared by grinding the vacuum-melted ingot and subjecting the ingot to electron-beam (EB) re-melting, to give a cylindrical ingot. It is then processed by forging and hot rolling into a plate material having a thickness of 6 mm, which is then solution-treated under a condition of 1050° C. for ½ hour, to give a plate material having a thickness of 6 mm. These materials were used as samples in Coriou corrosion test, a test simulating intergranular corrosion in nitric acid solution containing higher-concentration metal ions, and also in slow strain rate test (SSRT) and CBB test, tests simulating stress corrosion cracking in high-temperature high-pressure water. In the slow strain rate test and the CBB test, which simulates neutron irradiation-induced precipitation, the samples were sensitized at 620° C. for 100 hours before the tests.

TABLE 1 No. C Si Mn P S Ni Cr Ti B(ppm) N O A 0.001 0.02 0.05 0.002 0.001 20.4 24.2 0.20 0.2 0.0025 0.0022 B 0.001 0.05 0.08 0.001 0.001 19.7 24.8 0.18 0.4 0.0031 0.0035 C 0.002 0.04 0.04 0.001 0.001 20.8 23.9 0.19 1.3 0.0019 0.0032 D 0.001 0.06 0.06 0.002 0.002 21.4 24.5 0.22 2.5 0.0028 0.0030 E 0.001 0.02 0.05 0.001 0.001 19.2 24.7 0.20 4.2 0.0037 0.0024 F 0.002 0.02 0.08 0.002 0.001 20.1 25.3 0.21 7.4 0.0033 0.0038 G 0.002 0.01 0.09 0.003 0.002 21.6 24.7 0.21 12.6 0.0024 0.0024 H 0.001 0.05 0.08 0.003 0.001 10.2 18.5 0.19 0.7 0.0032 0.0031 I 0.002 0.01 0.07 0.002 0.002 14.6 18.2 0.19 0.5 0.0036 0.0040 J 0.001 0.06 0.06 0.004 0.001 21.6 32.1 0.22 0.6 0.0027 0.0037 K 0.002 0.07 0.07 0.001 0.001 35.3 25.2 0.18 1.6 0.0033 0.0025 L 0.002 0.04 0.08 0.002 0.002 20.9 24.4 0.02 0.4 0.0026 0.0043 M 0.003 0.05 0.10 0.002 0.001 19.5 25.6 0.01 5.4 0.0041 0.0030 N 0.004 0.22 0.26 0.008 0.004 12.5 22.2 0.03 1.3 0.0035 0.0049 O 0.016 0.03 0.12 0.003 0.003 21.1 24.8 0.21 0.8 0.0025 0.0023 P 0.002 0.01 0.05 0.016 0.017 20.7 24.5 0.01 1.0 0.0027 0.0039 Q 0.001 0.08 0.21 0.009 0.001 21.8 25.8 0.18 1.2 0.0191 0.0126 R 0.003 0.74 0.11 0.002 0.001 35.2 25.7 0.02 0.7 0.0039 0.0055 S 0.002 0.05 2.21 0.003 0.002 34.7 25.1 0.18 1.5 0.0088 0.0043

In the Coriou corrosion test the sample was immersed in 500 ml of 8N boiling nitric acid solution containing 1.0 g/L of Cr⁶+ion for 24 hours repeatedly for four times, while the solution was exchanged, and the weight loss was determined and the corrosion rate and others calculated. The slow strain rate test was carried out by using a test piece having a gauge section diameter of 3 mm and a distance between marked points of 20 mm under the condition of a deformation speed of 0.5 μm/min in high-temperature high-pressure water (saturated oxygen concentration: 8 wt ppm, 70 kgf/cm², 290° C.).

The CBB test was carried out by using a test piece having a thickness of 2 mm, a width of 10 mm and a length of 50 mm and also using the jig shown in FIG. 2 in an autoclave and the jig, while the sample was immersed in high-temperature high-pressure water (saturated oxygen concentration: 8 wt ppm, 70 kgf/cm², 290° C.) for 500 hours. The test piece 1 was held between holders 3, together with graphite fiber wool 2 for spacing; bolts were inserted into bolt holes 4; and the test piece was tightened between the curved-surface holders 3. In the present embodiment, each holder has a region having a curvature of 100R. After immersion, the test piece was separated, and cracking of the test piece was evaluated by observation of the cross section.

The intergranular corrosion resistance in the higher-concentration metal ion-containing boiling nitric acid was evaluated in the Coriou corrosion test according to the following criteria:

-   Excellent: Corrosion rate: 3.00 g/(m² h) or less, and intergranular     corrosion depth: 25 μm or less. -   Favorable: Corrosion rate: 3.00 g/(m² h) or less and intergranular     corrosion depth: more than 25 μm and 30 μm or less. -   Unfavorable: Corrosion rate: more than 3.00 g/(m² h), or     intergranular corrosion depth: more than 30 μm.

The stress corrosion cracking resistance in the high-temperature high-pressure water was evaluated in the SSRT corrosion test and the CBB test, according to the following criteria:

-   Excellent: Fracture time: 250 hours or more and intergranular     fracture ratio: 20% or less in SSRT corrosion test, and no     intergranular corrosion generation in CBB test. -   Favorable: Fracture time: 250 hours or more and intergranular     fracture ratio: more than 20% and 25% or less in SSRT corrosion test     and no intergranular corrosion generated in CBB test. -   Unfavorable: Fracture time: less than 250 hours or intergranular     fracture ratio: more than 25% in SSRT corrosion test and     intergranular corrosion generated in CBB test.

Test results after evaluation are summarized in Table 2. The samples of steel numbers A to D and steel numbers K to L each containing the following elements at the indicated contents: C, at 0.005 wt % or less; Si, at 0.5 wt % or less; Mn, at 0.5 wt % or less; P, at 0.005 wt % or less; S, at 0.005 wt % or less; Ni, at 15.0 to 40.0 wt %; Cr, at 20.0 to 30.0 wt %; N, at 0.01 wt % or less; 0, at 0.01 wt % or less; and B, at 3 wt ppm or less, and thus represent the inventive examples. On the other hand, the samples of steel numbers E to G and M represent samples of Comparative Examples and contain B in an amount of more than 3 wt ppm. The samples of steel numbers H and I represent samples of Comparative Examples and contain Cr in an amount of less than 20.0 wt % and Ni in an amount of less than 15.0 wt %. The sample of steel number J represents a sample of Comparative Example and contains Cr in an amount of more than 30.0 wt %. The samples of steel numbers N to Q represent samples of Comparative Example and contain C, at more than 0.005 wt %; P, at more than 0.005 wt %; S, at more than 0.005 wt %; N, at more than 0.01 wt %: or 0, at more than 0.01 wt %. The sample of steel numbers R and S represent samples of Comparative Example, and contain Si at more than 0.5 wt % or Mn at more than 0.5 wt %. Table 2 shows that the samples of steel numbers A to D and steel numbers K to L have intergranular corrosion resistance and stress corrosion cracking resistance more favorable than those of the samples of steel numbers E to J and steel numbers M to S.

The relationship between the corrosion rate in Coriou corrosion test and the B content of the samples of steel numbers A to G, L and M corresponding to so-called 25Cr-20Ni steels (steel containing Cr at approximately 25 wt % and Ni at approximately 20 wt %, C, at 0.005 wt % or less; Si, at 0.5 wt % or less; Mn, at 0.5 wt % or less; P, at 0.005 wt % or less; and S, at 0.005 wt % or less), among the samples of steel numbers A to S, is shown in FIG. 1A. Alternatively, the relationship between the intergranular corrosion depth in Coriou corrosion test and the B content of the sample of steel numbers A to G, L and M corresponding to 25Cr-20Ni steel is shown in FIG. 1B. FIGS. 1A and 1B show that a B content of more than 3 wt ppm leads to drastic increase in corrosion rate and intergranular corrosion depth. It also shows that, when the B contents are the same, the corrosion rate and the intergranular corrosion depth are reduced further by addition of Ti.

TABLE 2 CORIOU TEST CBB TEST CORROSION INTERGRANULAR SSRT TEST PRESENCE OF STEEL RATE CORROSION GRAIN- BOUNDARY BREAKAGE INTERGRANULAR NO. (g/m² · h) DEPTH (μm) FRACTURE RATE (%) TIME (h) CORROSION REMARKS A 1.15 2 0 385 NO INVENTIVE EXAMPLE B 1.74 4 0 360 NO INVENTIVE EXAMPLE C 2.16 7 2 315 NO INVENTIVE EXAMPLE D 2.63 9 5 285 NO INVENTIVE EXAMPLE E 4.79 31 30 225 YES COMPARATIVE EXAMPLE F 7.83 54 55 190 YES COMPARATIVE EXAMPLE G 9.05 85 85 135 YES COMPARATIVE EXAMPLE H 3.63 31 40 185 YES COMPARATIVE EXAMPLE I 4.29 48 50 150 YES COMPARATIVE EXAMPLE J 3.22 34 0 325 YES COMPARATIVE EXAMPLE K 2.68 11 0 350 NO INVENTIVE EXAMPLE L 1.97 8 0 320 NO INVENTIVE EXAMPLE M 8.54 51 40 210 YES COMPARATIVE EXAMPLE N 4.06 44 35 175 YES COMPARATIVE EXAMPLE O 5.21 49 55 180 YES COMPARATIVE EXAMPLE P 4.56 42 50 160 YES COMPARATIVE EXAMPLE Q 3.88 45 45 185 YES COMPARATIVE EXAMPLE R 3.74 36 60 155 YES COMPARATIVE EXAMPLE S 4.65 38 65 198 YES COMPARATIVE EXAMPLE

(Experiment 2)

Plate materials having a thickness of 6 mm were prepared by using the samples of steel numbers B, K and L shown in Table 1 under the various conditions shown in Table 3. The sample of production lot number 1 was an ingot obtained by vacuum induction melting (VIM) and casting into a mold under vacuum, while the samples of other production lot numbers (2 to 8) were subjected additionally to electron-beam re-melting (EB). Each plate material finished by forging and hot rolling was subjected to solution treatment and additionally to thermomechanical treatment (cold work-recrystallization or cold work-carbide precipitation-recrystallization) (the thickness of the plate having a different cold-working ratio was adjusted during solution treatment). The samples were analyzed in the Coriou corrosion test simulating intergranular corrosion in higher-concentration boiling nitric acid solution containing highly oxidizing metal ion, and the slow strain rate test (SSRT) and the CBB test simulating stress corrosion cracking in high-temperature high-pressure water. The samples were sensitized at 620° C. ×100 hours, in the slow strain rate test and the CBB test simulating neutron irradiation-induced precipitation.

TABLE 3 PRODUC- INGOT SOLUTION TREATMENT THERMOMECHANICAL TREATMENT STEEL TION LOT FORM- CONDITION REHEATING COLD ROLLING PRECIPITATION RECRYSTALLIZA- NO. NUMBER ATION (×½ h) CONDITION RATIO TREATMENT TION CONDITION REMARKS B 1 VIM 1050° C. — — — — B 2 VIM-EB 1050° C. — — — — B 3 VIM-EB 1050° C. 700° C. × 1 h — — — B 4 VIM-EB 1050° C. — 35% — 800° C. × 1 h B 5 VIM-EB 1050° C. — 60% — 800° C. × 1 h B 6 VIM-EB 1050° C. — 85% — 800° C. × 1 h B 7 VIM-EB 1050° C. — 60% 600° C. × 10 h  850° C. × 10 h B 8 VIM-EB 1050° C. — 60% 450° C. × 15 h  850° C. × 10 h K 1 VIM 1050° C. — — — — K 2 VIM-EB 1050° C. — — — — K 3 VIM-EB 1050° C. 700° C. × 1 h — — — K 4 VIM-EB 1050° C. — 35% — 750° C. × 1 h K 5 VIM-EB 1050° C. — 60% — 750° C. × 1 h K 6 VIM-EB 1050° C. — 85% — 750° C. × 1 h K 7 VIM-EB 1050° C. — 60% 600° C. × 10 h 800° C. × 1 h K 8 VIM-EB 1050° C. — 60% 450° C. × 15 h 800° C. × 1 h L 1 VIM 1050° C. — — — — L 2 VIM-EB 1050° C. — — — — L 3 VIM-EB 1050° C. 700° C. × 1 h — — — L 4 VIM-EB 1050° C. — 35% — 800° C. × 1 h L 5 VIM-EB 1050° C. — 60% — 800° C. × 1 h L 6 VIM-EB 1050° C. — 85% — 800° C. × 1 h L 7 VIM-EB 1050° C. — 60% 600° C. × 10 h 850° C. × 1 h L 8 VIM-EB 1050° C. — 60% 450° C. × 15 h 850° C. × 1 h

Test results are shown in Table 4. Table 4 shows that, when the samples of steel number B, K or L are processed by the treatments of production lot number 3, 5 or 7, i.e., solution-treated at 1050° C. for 30 minutes and then reheated at 700° C. for 1 hour (production lot number 3), or additionally cold-worked at a cold rolling ratio of 60% in the thermomechanical treatment step (production lot number 5), or precipitation-treated 600° C. for 10 hours in the thermomechanical treatment step after cold working and before recrystallization treatment (production lot number 7), intergranular corrosion was not observed in the CBB test, indicating the more favorable intergranular corrosion resistance can be obtained for the samples above than for those obtained by the treatments of other production lot numbers (1, 2, 4, 6 and 8).

TABLE 4 CORIOU TEST CBB TEST PRODUC- CORROSION INTERGRANULAR SSRT TEST PRESENCE OF STEEL TION LOT RATE CORROSION BREAKAGE GRAIN- BOUNDARY INTERGRANULAR NO. NUMBER (g/m²· h) DEPTH (μm) TIME (h) FRACTURE RATE (%) CORROSION REMARKS B 1 2.06 8 365 0 NO B 2 1.74 4 360 0 NO B 3 2.11 7 370 0 NO B 4 3.12 25 245 30 YES B 5 1.81 3 375 0 NO B 6 2.88 34 265 35 YES B 7 1.87 5 345 0 NO B 8 3.66 41 210 45 YES K 1 2.55 13 340 0 YES K 2 2.68 11 350 0 NO K 3 2.28 7 365 0 NO K 4 4.15 34 250 25 YES K 5 2.23 6 370 0 NO K 6 3.56 40 260 30 YES K 7 2.16 8 375 0 NO K 8 4.44 42 245 50 YES L 1 2.27 11 285 0 YES L 2 1.97 8 320 0 NO L 3 2.04 10 290 0 YES L 4 3.48 40 245 35 YES L 5 1.95 9 335 0 NO L 6 3.38 47 235 40 YES L 7 2.19 12 340 0 NO L 8 3.86 39 260 45 YES

As described above in detail, an aspect of the present invention is an austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance, comprising: C: 0.005 wt % or less; Si: 0.5 wt % or less; Mn: 0.5 wt % or less; P: 0.005 wt % or less; S: 0.005 wt % or less; Ni: 15.0 to 40.0 wt %, Cr: 20.0 to 30.0 wt %, N: 0.01 wt % or less; O: 0.01 wt % or less; and the balance of Fe and inevitable impurities, wherein the content of B included in the inevitable impurities is 3 wt ppm or less.

It is possible, in the configuration, to reduce intergranular corrosion and prevent stress corrosion cracking sufficiently by adjusting the B content to 3 wt ppm or less.

It is also possible to prevent precipitation of Cr based carbides by adjusting the C content to 0.005 wt % or less. It is also possible to cause deoxidization by adjusting the Si content to 0.5 wt % or less. It is also possible to reduce generation of 8-ferrite and work induced phase transformation by adjusting the Mn content to 0.5 wt % or less. It is also possible to reduce deterioration in intergranular corrosion resistance and stress corrosion cracking resistance by adjusting the P content to 0.005 wt % or less. It is also possible to reduce deterioration in intergranular corrosion resistance, stress corrosion cracking resistance and pitting corrosion resistance by adjusting the S content to 0.005 wt % or less.

It is also possible to stabilize the austenitic structure and suppress intergranular corrosion and stress corrosion cracking by adjusting the Ni content to 15.0 wt % or more. It is also possible to reduce the cost by adjusting the Ni content to 40.0 wt % or less. It is also possible by adjusting the Cr content to 20.0 wt % or more to assure sufficient corrosion resistance, for example, under an overpassivation corrosion environment under a corrosion environment for a boiling heat transfer surface in a high-concentration nitric acid solution containing highly oxidizing metal ions such as in reprocessing plants and under an environment in high-temperature high-pressure water exposed to neutron irradiation such as in light water reactor cores. It is also possible by adjusting the Cr content to 30.0 wt % or less to suppress precipitation of a Cr-rich brittle phase. It is also possible to reduce deterioration in intergranular corrosion resistance and stress corrosion cracking resistance by adjusting the N and O contents respectively to 0.01 wt % or less.

The total content of C, P, S, N and O in the austenitic stainless steel is preferably 0.02 wt % or less for obtaining favorable intergranular corrosion resistance and stress corrosion cracking resistance.

The austenitic stainless steel preferably further comprises Ti, wherein the content of Ti is in an amount stoichiometrically equivalent to or more than the total amount of C, P, S, N and O, because the intergranular corrosion-causing impurity elements such as C, N, P, S and O can be removed completely by conversion to Ti based carbides, nitrides or other compounds such as TiC, TiN, FeTiP, TiS and TiO₂.

Another aspect of the present invention is a method of producing an austenitic stainless steel material excellent in intergranular corrosion resistance and stress corrosion cracking resistance. The method comprises: a step of hot working an ingot having a chemical composition of the abovementioned stainless steel; and a step of solution treating a stainless steel material obtained by the hot working; wherein the solution treatment step comprises: a substep of heating the stainless steel material at a heat treatment temperature in a first temperature range of 1000 to 1150° C. for 1 minute or more; and then, a substep of cooling the stainless steel material from the heat treatment temperature in the first temperature range to ordinary temperature by rapid or natural cooling.

It is possible in the configuration, to uniformize the austenite phases by solution treatment and increase the effect to improve intergranular corrosion resistance and stress corrosion cracking resistance by restricting the chemical composition of the austenitic stainless steel.

Yet another aspect of the present invention is a method of producing an austenitic stainless steel material excellent in intergranular corrosion resistance and stress corrosion cracking resistance. The method comprises: a step of hot working an ingot having a chemical composition of the abovementioned stainless steel; and a step of solution treating a stainless steel material obtained by the hot working; wherein the solution treatment step comprises: a substep of heating the stainless steel material at a heat treatment temperature in a first temperature range of 1000 to 1150° C. for 1 minute or more; a substep of cooling the stainless steel material from the heat treatment temperature in the first temperature range by rapid or natural cooling; a substep of heating the stainless steel material at a heat treatment temperature in a second temperature range of 650° C. or higher for 10 minutes or more after the cooling, and then, a substep of cooling the stainless steel material from the heat treatment temperature in the second temperature range to ordinary temperature by rapid or natural cooling.

It is possible in the configuration to uniformize the austenite phases by solution treatment in the first temperature range, facilitate generation of the Ti based deposit by heat treatment after solution treatment in the second temperature range, and increase the effect of improving intergranular corrosion resistance and stress corrosion cracking resistance by restriction of the chemical composition of the austenitic stainless steel.

The method of producing an austenitic stainless steel material preferably, after the solution treatment step, further comprises: a step of cold working the stainless steel material at a working ratio of 40% or more and less than 75%; and then a step of recrystallization treatment by heating the stainless steel material at a heat treatment temperature in a temperature range of 700° C. or higher for 10 minutes or more.

It is possible in the configuration to introduce dislocations as precipitation sites sufficiently during the cold working and prevent strain induced phase transformation from austenitic phase to martensite phase by excessive working. It is thus possible to prevent increase in difficulty in industrial working and to obtain a uniform austenite structure by the subsequent recrystallization treatment. It is also possible to obtain a uniform austenite structure by the recrystallization treatment and thus to obtain favorable intergranular corrosion resistance and stress corrosion cracking resistance.

The method of producing an austenitic stainless steel preferably, between the cold working step and the recrystallization treatment step further comprises: a step of deposit precipitation in strain aging by heating the stainless steel material at a heat treatment temperature in a range of 500 to 650° C. for 30 minutes or more.

It is possible in the configuration to disperse carbides and other precipitations efficiently in strain aging after the cold working and before the recrystallization treatment.

Favorable embodiments of the present invention, which have been described so far, are nothing but typical embodiments and do not restrict the present invention, and the typical configuration and others may be altered as needed in design. In addition, the operations and the effects described above in the embodiments of the invention are nothing but list of the most favorable operations and effects provided by the present invention, and not limited to those described in the embodiments of the present invention.

INDUSTRIAL APPLICABILITY

It is possible by using the austenitic stainless steel according to the present invention to improve both intergranular corrosion resistance and stress corrosion cracking resistance reliably, both under a corrosion environment for a boiling heat transfer surface in a high-concentration nitric acid solution containing highly oxidizing metal ions and under the environment in high-temperature high-pressure water exposed to neutron irradiation. 

1. An austenitic stainless steel comprising Fe and: C: 0.005 wt % or less; Si: 0.5 wt % or less; Mn: 0.5 wt % or less; P: 0.005 wt % or less; S: 0.005 wt % or less; Ni: 15.0 to 40.0 wt %, Cr: 20.0 to 30.0 wt %, N: 0.01 wt % or less; O: 0.01 wt % or less; and inevitable impurities, wherein a content of B in the total weight of the steel is 3 wt ppm or less.
 2. The austenitic stainless steel according to claim 1, wherein the total content of C, P, S, N and O is 0.02 wt % or less.
 3. The austenitic stainless steel according to claim 2, further comprising Ti, wherein a content of Ti is an amount stoichiometrically equivalent to or more than the total content of C, P, S, N and O.
 4. A method of producing an austenitic stainless , the method comprising: hot working an ingot having a chemical composition of the stainless steel according to claim 1; and solution treating the stainless steel material obtained by the hot working; wherein the solution treating comprises: heating the stainless steel material at a heat treatment temperature in a first temperature range of 1000 to 1150° C. for 1 minute or more; and then, cooling the stainless steel material from the heat treatment temperature in the first temperature range to ordinary temperature by rapid or natural cooling.
 5. A method of producing an austenitic stainless steel, the method comprising: hot working an ingot having a chemical composition of the stainless steel according to claim 1; and solution treating the stainless steel material obtained by the hot working; wherein the solution treating comprises: heating the stainless steel material at a heat treatment temperature in a first temperature range of 1000 to 1150° C. for 1 minute or more; cooling the stainless steel material from the heat treatment temperature in the first temperature range by rapid or natural cooling; heating the stainless steel material at a heat treatment temperature in a second temperature range of 650° C. or higher for 10 minutes or more after the cooling, and then, cooling the stainless steel material from the heat treatment temperature in the second temperature range to ordinary temperature by rapid or natural cooling.
 6. The method of producing an austenitic stainless steel material according to claim 4, after the solution treatment, further comprising: cold working the stainless steel material at a working ratio of 40% or more and less than 75%; and then heating the stainless steel material at a heat treatment temperature in a temperature range of 700° C. or higher for 10 minutes or more for recrystallization.
 7. The method of producing an austenitic stainless steel material according to claim 6, between the cold working and the recrystallization treatment, further comprising: heating the stainless steel material at a heat treatment temperature in a range of 500 to 650° C. for 30 minutes or more to deposit precipitation in strain aging. 